I picked DDfix 1.5.11 and it still asks for the frickin CD. Strange...
Ok well I have another trick up my sleeve here. here
is my thief2.exe. It may or may not stop it asking for a CD, I'm not sure. I know it doesn't ask me.
It might be illegal for me to post this, but you own the game and there's nothing one can do with thief2.exe on its own, right? So I'll leave this link intact unless a blue man comes along and tells me it's not OK.
Just replace your thief2.exe and hope it works.